Managing apple scab on ornamental trees and shrubs
- Apple scab is the most common disease of ornamental crabapples in Minnesota.
- Infected leaves have olive green to brown leaf spots.
- Leaves with many leaf spots turn yellow and fall off early.
- Early leaf loss weakens the tree when it occurs many years in a row.
- Planting disease resistant varieties is the best way to manage scab of flowering crabapples.
- Fungicides can be used to manage apple scab in susceptible varieties. Proper timing of sprays is needed for fungicides to control disease.
How to identify apple scab
- Leaf spots are round, olive-green in color and up to ½-inch across. They are velvet-like with fringed borders.
- As they age, leaf spots turn dark brown to black, get bigger and grow together.
- Leaf spots often form along the leaf veins.
- Leaves with many leaf spots turn yellow and drop by mid-summer.
- Infected fruit have olive-green spots that turn brown and corky with time.
- Fruit that are infected when very young become deformed and cracked as the fruit grows.
How does apple scab survive and spread?
Apple scab is caused by the fungus Venturia inaequalis. It infects crabapples and apples (Malus spp.), mountain ash (Sorbus spp.), pear (Pyrus communis) and Cotoneaster (Cotoneaster spp.).
The apple scab fungus has several host specific strains that can cause disease on one type of plant but not any other. For example, the strain of V. inaequalis that infects mountain ash will only infect other mountain ash trees and will not infect crabapple trees. Apple and crabapple trees are infected by the same strain of the apple scab fungus because the trees are in the same genus.
- The apple scab fungus overwinters on fallen diseased leaves.
- In spring, these fungi shoot spores into the air.
- Spores are carried by wind to newly developing leaves, flowers, fruit or green twigs.
- Spores need several hours of moisture on the plant surface in order to start new infections.
- These infections grow into spots that can produce more spores within 9 to 17 days.
- Spores are spread by wind, splashing rain or irrigation throughout the tree canopy or to neighboring trees, starting new infections.
- The infection cycle can repeat many times throughout the growing season whenever leaves remain wet long enough.
- Warm, rainy weather in the spring and summer creates ideal conditions for apple scab.
- Leaves with many leaf spots turn yellow and fall off early. This weakens the tree.
- Several years of early leaf loss can result in decreased growth, reduced bloom and increased susceptibility to winter injury.
How to manage apple scab
For control of apple scab on apple and edible crabapple varieties see pest management for the home apple orchard guide for home apple growers.
Planting disease resistant varieties is the best way to manage scab of flowering crabapples.
The following list highlights crabapple varieties that have shown strong scab resistance and are tolerant of Minnesota's low winter temperatures.
Apple scab resistant crabapple trees that are hardy in Minnesota:
M. halliana var. parkmanii
Malus x zumi
- Rake up and destroy fallen leaves before the first snowfall to get rid of places where the fungus can survive to re-infect the plant the following growing season.
- Instead of raking, leaves can be chopped with a mulching lawn mower.
- Fall lawn fertilizer applications will help breakdown leaves that have been chopped with a mulching lawn mower.
- In mulched areas, urea can be applied to chopped leaf litter to help with decomposition.
- Do not overcrowd plants. Use the mature size of the tree as a spacing guide.
- Prune crabapple trees so that the branches are spaced far enough apart to let air move through the branches and dry the leaves quickly.
- Remove upright suckers and water sprouts that have formed along the main trunk or within the canopy.
- This also improves air movement and sun penetration so leaves dry quickly after rain or dew.
When should fungicide sprays start?
Fungicides must be applied before leaf spots appear to successfully manage apple scab. Because apple scab spores are released so early in the growing season, fungicide sprays must begin when the first green leaf tips appear in spring.
How often should fungicide sprays be applied?
Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. (e.g. 7 to 10 days after spraying, you will need to spray again).
Use the shorter interval
- When the weather calls for frequent rain.
- Where scab has been a serious problem in past years.
Use the longer interval
- In dry weather with little or no rain.
- Where scab has not been a serious problem in past years.
When to stop fungicide sprays?
- Sprays should be repeated until the petals drop for crabapple. If the tree is healthy and free of leaf spots at this point, further treatments are unnecessary.
Do not apply fungicides to a diseased tree
Fungicides only protect healthy trees from becoming infected. Once leaf spots appear in the tree, fungicides will not control the disease. There is no point in spraying a tree that is already infected.
Can I spray my own tree?
Small trees can be treated by a home gardener if all instructions on the fungicide label are read and followed. Contact a certified arborist to apply fungicides to large trees.
The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used. Some products are registered for use on ornamental crabapples but are not safe to use on crabapple or apple fruit intended for eating. Always completely read and follow all instructions on the fungicide label.
Fungicide active ingredients to treat apple scab on ornamental crabapples:
- Sulfur/lime sulfur*
- Neem oil*
*Burning of plant tissue may be observed especially in times of high heat.
**Russeting (rough corky skin) may form on fruit with use of copper products.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
When treating fruits or vegetables, make sure the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018